The present invention relates generally to transfer cases for four-wheel drive vehicles and, more particularly, to such a transfer case having a gear reduction unit and an inter-axle differential integrated into a planetary gear assembly for providing a variety of part-time and full-time four-wheel drive operating modes.
Modernly, many light-duty and sport-utility vehicles are equipped with a power transfer unit, such as a four-wheel drive transfer case, for providing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, many four-wheel drive transfer cases are equipped with a gear reduction unit for providing a high-range (i.e., direct drive) and a low-range (i.e., reduced ratio drive) in conjunction with the four-wheel drive mode. Most commonly, the gear reduction units used in four-wheel drive transfer cases include either a layshaft arrangement or a planetary gear assembly. In addition, some transfer cases are also equipped with an interaxle differential mechanism for permitting torque proportioning and differential speed variations between the front and rear axles of the four-wheel drive vehicle. Reference may be made to U.S. Pat. No. 4,677,873 for disclosure of an exemplary four-wheel drive transfer case equipped with a planetary gear reduction unit and a planetary interaxle differential mechanism.
In an effort to minimize the overall size of transfer cases, it has been proposed to incorporate the gear reduction unit and the interaxle differential mechanism into a single planetary gear assembly. One such arrangement is described in U.S. Pat. No. 4,644,822 which discloses a transfer case having "back-to-back" first and second planetary gear sets having common components which can be selectively shifted as a unit to establish the different four-wheel drive modes. In addition, U.S. Pat. Nos. 4,677,875, 4,344,335, 4,215,593 and 4,805,484 each disclose a transfer case having a single planetary gear assembly which is operable to establish various drive modes through manipulation of one or more suitable shift mechanisms. While such prior art arrangements provide a compact construction, there is a continuing need to develop low cost, simplified alternatives which meet modern requirements for low noise and weight.